This invention generally relates to soft contact lenses and particularly those well known in the art as hydrogel lenses.
Conventional contact lenses are hard lenses polymerized using the principal co-monomer, methyl methacrylate. Hard lenses suffer from many disadvantages, however. One of the principal problems is discomfort to the wearer's eye after repeated periods of extended wear due to a condition called "oxygen starvation".
The epithelium of the cornea requires oxygen which is usually supplied from the oxygen dissolved in tears. However, because of the manner in which hard lenses conform to the contour of the eye, the flow of lacrimal fluid is greatly curtailed beneath the lens. This reduction in fresh lacrimal fluid is not desirable as it substantially reduces the contact of the eye with oxygen. Therefore, it is extremely important that the lens material itself be gas permeable. Hard lenses fail to admit sufficient oxygen and/or release sufficient carbon dioxide to maintain a healthy normal condition for the eye tissue and cornea covered, especially when the lens is worn continuously for extended periods of time. In other words, the conventional hard lenses cannot breathe sufficiently through the body of the lens. Due to these problems, many workers in the field have experimented with the production of soft contact lenses. Examples of such work for the purpose of improving hydrogel soft lenses is found in U.S. Pat. No. 3,957,362 to Mancini et al, U.S. Pat. No. 3,983,083 to Kaetsu et al and U.S. Pat. No. 3,988,274 to Masuhara et al and the disclosures in these patents are herewith incorporated by reference. The presently known soft contact lenses are made of hydrophilic polymers, the most common being hydroxyethyl methacrylate (known in the art as "HEMA").
The hydrogel soft contact lenses receive their flexibility from their capacity to absorb water. After machining and polishing the lenses, they are placed in a saline solution from which they absorb water and swell until equilibrium is obtained. This process allows hydrogel lenses to possess a high degree of hydration which is directly related to the mode of oxygen transport and the lenses are thereby able to provide the eye with sufficient oxygen.
An average percent hydration of the conventional soft lenses now available is approximately 35-45%. The hydration level is calculated by use of the following formula: ##EQU1## In short, the more water that the lens is able to absorb, the higher the hydration percent and therefore, the better the oxygen permeability of the lens.
A problem associated with highly hydrated hydrogel lenses is that although they are able to obtain satisfactory oxygen transport levels, since they are used in the swollen state, the molecular materials of their composition are markedly reduced in mechanical strength and the resulting lenses are extremely fragile. Due to this fragileness, the thickness of the lens must be increased and, therefore, these prior art soft lenses are ill-suited for the preparation of ultra-thin corneal lenses. By increasing the thickness of the lens, the gas permeability of the lens is thereby decreased forming a vicious cycle between gas permeability and strength.
In making an ultra-thin lens, the greater the strength and the greater the refractive index of the material used, the better the resulting thin lens.
The disclosed invention obviates the above deficiency in the prior art by providing a co-monomer mixture suitable for producing hydrogel contact lenses which have a superiorly high strength and refractive index, which can withstand sterilization, and, in addition, offer superior gas permeability. A further advantage of these materials, especially the more hydrophilic ones, lies in the fact that lenses made of these materials have a much lesser tendency to dehydrate while on the eye. These properties make the fabrication of and the wearing of an ultra-thin lens a practical reality.